Preheat oven to 350.
Saute onion and mushrooms in a approximately 2 tbsp. of butter, in a cast-iron dutch oven. Cook approx. 3 minutes.
Add chicken to dutch oven. Brown three minutes on each side. Add canned tomatoes and roasted red pepper.
In a small dish, whisk together mustard, Worcestershire sauce, vinegar, white pepper and cardamom. Pour over chicken thighs.
Cover and bake in oven for 45 minutes. Remove the lid 10 minutes prior to serving to allow some of the liquid to cook off and to allow sauce to thicken. (I added a tbsp. of corn starch, mixed with a tbsp. of water, to the dish to finish, and another tbsp. of butter.)
**I highly suggest serving this dish with mashed potatoes. The sauce this dish creates is AMAZING on potatoes.